1. Field of the Invention
The present invention relates to an apparatus for detecting the position of an object. More particularly, the present invention relates to an apparatus for detecting signals used to align two objects such as a mask and wafer in a semiconductor exposure system and the like.
2. Description of the Prior Art
In the prior art systems for aligning the mask and wafer with each other, the mask and wafer respectively having alignment marks thereon are disposed one above the other on a plane to be scanned and then scanned by the use of light to detect time intervals between the alignment marks, the time intervals being used to detect the positional relationship between the alignment marks.
With reference to FIG. 1, an alignment mark on a mask includes first and second alignment mark elements M1 and M2 parallel to each other on a scanning line A, and third and fourth alignment mark elements M3 and M4 parallel to each other on the same scanning line A and oppositely inclined relative to the scanning line A, all of which elements are inclined relative to the scanning line A by an angle .theta. of 45 degrees, as shown in FIG. 1(a). An alignment mark on a wafer includes first and second alignment mark elements W1 and W2 which are inclined oppositely relative to each other and also relative to the scanning line A by an angle .theta. of 45 degrees, respectively, as shown in FIG. 1(b). The alignment of the mask and wafer relative to each other will be carried out by overlapping the alignment marks one above the other as shown in FIG. 1(c).
U.S. Pat. No. 3,683,195 discloses an apparatus for scanning such alignment marks along the scanning line A by the use of an optical scanning mechanism which includes movable slit-like openings spaced apart from each other and extending in two different directions. One of the slit-like openings is used to detect the positional relationship, for example, among the marks M1, W1 and M2, while the other slit-like opening is utilized to detect the positional relationship among the remaining marks M3, W2 and M4. However, the prior art optical apparatus is disadvantageous in that it is essential to provide a plurality of slits resulting in a plurality of optical systems. There is also a problem that the detection light is less effectively utilized since it will illuminate the entire area to be observed with only part thereof devoted to detection.
U.S. Pat. No. 4,167,677 discloses a detection apparatus for scanning the surface of an object by the use of a light spot. The detection apparatus of this patent is disadvantageous in that if any foreign matter is on the scanning line or if a part of the alignment mark is broken off, there will occur failure in alignment or deterioration of detection accuracy. U.S. patent application Ser. No. 438,638 filed 11-3-82 discloses a detection apparatus which scans the alignment marks by the use of a bar-like beam inclined to conform to the orientation of one of the alignment marks, the inclination of the bar-like beam being changed to conform the orientation of the other alignment mark between the first and second scannings. The detection apparatus of this U.S. patent application requires a complicated changing mechanism with high accuracy and a prolonged period of time for alignment measurement.